1. Field of the Invention
The present invention relates to a power transmission unit for automotive vehicles, and more particularly to an air breather plug assembly adapted for use in a casing of the power transmission unit.
2. Discussion of the Background
In general, such an air breather plug assembly as described above is mounted on the upper portion of a transmission or trans-axle casing to allow the flow of air out from the interior of the casing to the exterior and to prevent leakage of the lubricating oil out from the casing. As disclosed in Japanese Utility Model Early Publication No. 57-158519, the transmission or trans-axle casing has been provided therein with an annular isolated space for preventing the lubricating oil from being directly splashed on the inner end of the plug assembly. In recent years, however, the transmission or trans-axle casing is formed to contain a change-speed gearing driven by a high performance engine and a power transfer mechanism for four-wheel drive and is filled with a large amount of lubricating oil for sufficient lubrication of the change-speed gearing and the power transfer mechanism. For this reason, an increase of lubricating oil picked up by rotation of the change-speed gearing and rise of the oil temperature will cause leakage of the lubricating oil out from the casing.